Transfer molding using an epoxy resin composition is typically adopted as an economically advantageous means for encapsulating semiconductor devices, such as LSIs, ICs, and transistors.
Recently, resin-encapsulated LSIs are directly soaked in a solder bath for surface mounting. In the surface mounting process, the encapsulating material is exposed to a temperature of 200.degree. C. or even higher. The water absorbed in the encapsulating material is accordingly expanded and causes cracks in the encapsulating package of the semiconductor devices.
The epoxy resin encapsulating material is accordingly required to have low moisture absorption characteristics and improved cracking resistance. A widely used encapsulating material includes glycidyl ether of o-cresol novolak as an epoxy resin and phenol novolak as a curing agent. This widely used encapsulating material must be stored in moisture-resistant packaging to avoid the above problems. To solve the above mentioned problems, especially to obtain low moisture absorption characteristics, low viscosity epoxy resins which can contain large amount of fillers have been developed and used for practical applications; for example, a glycidyl ether epoxy resin having a tetramethylbiphenyl skeleton.
To improve the physical properties of the encapsulating material, the technique of improving the mechanical properties of cured resin is effective. A known method of improving the mechanical properties of cured objects of thermosetting resin controls the molecular orientation in the cured objects. There is a prior art document which is related to a stilbene skeleton-containing epoxy resin. It has been reported that polymerization of a liquid crystal properties-possessing epoxy compound in the liquid crystal state in the presence of a small quantity of catalyst yields a crosslinked body which retains the liquid crystal structure and that polymerization in an electric field of specific conditions serves to orientate the liquid crystal domain as seen from page 182 in the Proceedings of the 3rd Next-generation Industrial Infrastructure Technology Symposium (1985). In this report, epoxidized 4,4'-dihydroxy-&A-cyanostilbene is shown as an example of the compound having liquid crystal properties.
Some epoxy compounds including the rod-like structure of carbon--carbon double bond, carbon-nitrogen double bond, nitrogen--nitrogen double bond, or the like, which is different from the structure of the present invention, have been proposed as compounds possessing excellent physical properties in Japanese Patent Laid-open No. S-64-56721 and Japanese Patent Laid-open No. H-1-85215.
A method for manufacturing structural members has been proposed, in which the structural members were cured with using a compound having a liquid crystal properties-donating component in the molecular structure thereof, while maintaining the liquid crystal structure, thereby improving the mechanical properties as seen from U.S. Pat. No. 4,762,901, German Patent No. 3,622,613, and Japanese Patent Laid-open No. S-63-23931. A glycidyl ether of bisphenol compound having a non-substituted or alkyl group-substituted stilbene skeleton, or preferably having a stilbene skeleton with symmetrically substituted methyl groups, is given just as an example of such liquid crystal properties-containing compounds with other known compounds. Some stilbene bisphenol compounds having identical aryl substituents bound to the carbon-carbon double bond have also been reported previously. Preparation and physical properties of the compounds, such as 4,4'-dihydroxystilbene, 4,4'-dihydroxy-3,3'-dimethylstilbene, 4,4'-dihydroxy-3,3',5,5'-tetramethylstilbene, have been disclosed (von Rolf H. Sieber, Liebigs Ann. Chem. 730, 31-46(1969)). A method of preparing 4,4'-dihydroxy-&A-methylstilbene has been described in METHODEN DER ORGANISCHEN CHEMIE (HOUBEN-WEYL) BAND IV/1c Phenol Teil 2 P1034.
Epoxy resins having liquid crystal properties or the rod-like structure for the improved mechanical properties have also been proposed as seen from Japanese Patent Laid-open No. H-2-275872. Examples of hydroxyl group-containing compounds for epoxidation are 4,4'-dihydroxy-&A-methylstilbene, 4,4'-dihydroxystilbene, 4,4'-dihydroxy-3,3',5,5'-tetrabromostilbene, and 4,4'-dihydroxy-3,3',5,5'-tetramethylstilbene. Resin compositions of the epoxy resin having the liquid crystal properties-developing functional group or the rod-like structure and the compound having active hydrogen have also been proposed for improving the physical properties as seen from Japanese Patent Laid-open No. H4-233933, U.S. Pat. No. 5,292,831, U.S. Pat. No. 5,270,405, U.S. Pat. No. 5,270,404, and U.S. Pat. No. 5,266,660. In the specifications of these patents or patent application, orientation of molecules in a cured object with the given epoxy compound has been proposed in order to improve the physical properties of the cured object.
Conventional resins for encapsulant, such as, for example, encapsulating materials comprising a glycidyl ether of o-cresol novolak, have substantially-balanced heat resistance and molding properties, but possess poorer physical properties as encapsulants than those of biphenyl epoxy resins. The biphenyl epoxy resins have low moisture absorption characteristics and excellent physical properties as the encapsulating material for surface mounting, but have undesirably low heat resistance and which results in package cracks under high humidity condition.
The conventional stilbene epoxy resins show excellent curing properties but have high melting points and poorer working properties in the process of mixing the epoxy resin component with inorganic fillers or in the molding process. For example, glycidyl ethers of 4,4'-dihydroxystilbene, 4,4'-dihydroxy-3,3'-dimethylstilbene, and 4,4'-dihydroxy-3,3',5,5'-tetramethylstilbene have high melting points of 208 to 215.degree. C., 150.degree. C., and 151.degree. C., respectively, and suffer from rather bad working properties. In fact, the epoxy resin of 4,4'-dihydroxystilbene can not be used for encapsulating semiconductors under the conditions practiced with the current production equipment.
Semiconductor-encapsulating materials including thermosetting resins are formed to packages within a time period of 60 through 90 seconds. The cured objects of such material have a three-dimensional crosslinked structure without any specific molecular orientation as disclosed in the prior art described above. The prior arts above have been proposed to further improve the physical properties of cured objects by introducing a specific orientation to the cured structure. To introduce the liquid crystal state or the specific molecular orientation to cured objects, molding under the specified temperature conditions or external control of the molding conditions by means of an electric field or magnetic field is required. The cured objects having the specific molecular orientation have different strengths and thermal expansion coefficients according to the direction of molding. When stress is applied in the form of tests, such as the solder heat resistance test after water absorption, it is concentrated on specific portions of a package having low strength and eventually causes cracks in the package.
The cured object prepared from the resin composition according to the present invention is only required to have a cured structure of substantially equivalent to that of the conventional thermosetting epoxy resin composition, and is not a cured object including a specific molecular orientation and having the deteriorating physical properties according to the direction.
Another technique to further accelerate the orientation of molecules are proposed in which the epoxy compound having a liquid crystal properties-developing group is previously reacted with the compound having active hydrogen. This technique heightens the degree of polymerization of the resin composition, and accordingly increases the melt viscosity of the resin composition including inorganic fillers, which causes difficulties in the molding process. The present invention does not require any specific pre-reaction of the epoxy component with the epoxy curing component, and accordingly has no specific cured structure observed as a result of such pre-reaction.
The conventional stilbene epoxy resin, having a high melting point and low solubility in an organic solvent, is hardly applied to laminated sheets, composite material, or coating material.